Warat Kongkitkul

Simple Dynamic Hammer for Evaluation of Physical Conditions of Pavement Structures

Even though low-volume roads usually serve as secondary roads on the entire road network, they are of great importance for the distribution of goods to a number of rural areas in Thailand. However, the budgets for evaluation of the physical condition of the pavement structure of roads in this category and for their subsequent maintenance are surprisingly limited. In addition, the number of specialists required to perform visual inspections is small, and simple static tests on road surfaces are time-consuming and thus expensive. Consequently, it is difficult, if not impossible, to evaluate the physical conditions of pavement structures at a sufficient number of locations. To mitigate this, it becomes necessary to develop a simple test device and a simple means of test analysis to evaluate the physical condition of the pavement structure. This paper introduces a dynamic hammer and discusses a procedure for analysis of the test data. Experiments were performed on both asphalt-paved and unpaved surfaces, which are usually used for low-volume roads. In addition, plate load tests were performed in parallel to compare the test results and validate the analytical framework. It was found that the test device and the framework for analysis of the test data presented in this paper are relevant for use, in particular because of the simplicity of device production, the ease of device use, the time-effectiveness of test performance, and the procedures used for data analysis. Therefore, maintenance can be performed at sufficient numbers of locations of low-volume roads to keep them in good physical condition for serving the entire road network.

Post-Construction Time History of Tensile Force in Geogrid Arranged in a Full-Scale High Wall

To confirm a very low possibility of tensile rupture until the end of specified life time of polymer geogrids arranged in a full-scale reinforced soil retaining wall (21 m-high with a slope of 1:0.3 in V:H) constructed to support a taxi way of an airport, time histories of tensile force in the reinforcement were estimated based on those of measured tensile strain in the reinforcements in the wall. To this end, tensile tests were performed on the geogrids and their elasto-viscoplastic properties were evaluated. A constitutive model was developed based on the test results. The model was validated by that the model can accurately predict the tensile load-strain-time behaviour of the geogrids when subjected to arbitrary loading histories. The time histories of tensile force in the geogrids in the wall estimated from the measured tensile strains based on the model indicate that, even in the most severe case among those analysed, the tensile force will increase with time to a very limited extent, towards a value significantly lower than the tensile rupture strength at the end of life time. This result indicates that eventual creep rupture of the geogrids in the wall is not likely unless significant degradation takes place.

Efficiency of Rice Husk Ash as Cementitious Material in High-Strength Cement-Admixed Clay

The potential and efficiency of using rice husk ash (RHA) to add up or partially replace Portland cement in deep cement mixing technique are examined. A series of unconfined compression tests on cement-RHA-stabilized clay are conducted to investigate the influence of RHA on the mixture properties. Special attention is paid to its efficiency for increasing the strength by partial cement replacement to obtain high-strength soil cement, and it is compared with fly ash. Test results indicate that up to 35% of RHA could be advantageously added up to enhance the strength if the cement content in the mixture is larger than 10%. The RHA enhances the strength of cement-admixed clay by larger than 100% at 28 days. For curing time of 14 and 28 days, the RHA exhibits higher efficiency on Portland cement replacement when the cement and overall cementitious contents are not less than 20 and 35%, respectively. The optimum condition for high-strength mixture is achieved when RHA is added to the 20% cement content mixture. When compared with fly ash of similar grain size, the efficiency of RHA is higher when the content to be added is greater than 15%. This indicates the suitability of RHA for use in high-strength soil-cement.

Evaluation of Guy Anchorage Strength in Clay for Transmission Tower

Due to the growth of the economy in Thailand, the electricity transmission system must be regularly maintained so that electricity is effectively and safely distributed for uses in daily life and industries. Natural disasters often cause damage to transmission towers. In addition, some damage has occurred due to vandalisms and soil excavations near transmission towers. Foundation scouring and tilting typically occur at transmission towers. In practice, repair of damage to a transmission tower typically begins with transferring the foundation pressure of the damaged tower to other firm foundation soil using a guy fixed to a guy anchor. The transmission tower is then restored to its original condition. In this study, a series of field anchorage strength tests were performed in the Bangkok metropolitan region which is well-known for its very soft to soft clay deposits. Additionally, Kunzelstab penetration tests were performed to evaluate the undrained shear strengths of the clay with depth at the test locations. The measured ultimate anchorage strengths were then compared with values estimated theoretically on the basis of measured undrained shear strengths. The measured values were found to be approximately 70 % of the theoretical values. This was probably due to the clay disturbance caused by anchor installation. Based on this fact, the ultimate anchorage strengths were then estimated for other clay conditions.

Hypoplastic Model for Simulation of Deformation Characteristics of Bangkok Soft Clay with Different Stress Paths

This paper presents the development of hypoplastic model for simulating deformation characteristics of Bangkok Soft Clay. The results from drained triaxial tests of Bangkok Soft Clay were simulated by the new hypoplastic model with applied stress path of 0–180 degrees in p–q stress space. The proposed hypoplastic model has satisfy performance to predict the deformation characteristics of Soft Bangkok Clay. The stiffness of proposed model varied according to load and unloading condition and different applied stress path.

INELASTIC DEFORMATION OF SAND REINFORCED WITH DIFFERENT REINFORCING MATERIALS

Geomaterial as well as polymer geosynthetic reinforcement are known to have significant visco-plastic property. A series of plane strain compression tests were performed on air-dried Toyoura sand reinforced with four grid types of reinforcement to evaluate the effects of the strength and deformation characteristics (inextensible or extensible; and nearly elastic or highly non-linear elasto-viscoplastic), surface conditions (smooth or rough) and the degree of in-plane dispersion of reinforcement. The effect of reinforcement stiffness on the pre-peak stiffness and peak strength of reinforced sand was not significant, while the effects of the surfaced roughness and the degree of in-plane dispersion were more significant. Significant viscous effects on the vertical stress - vertical strain behaviour of reinforced sand were observed, which were due mainly to the viscous property of sand. The tensile load in the geogrid reinforcement arranged in the sand subjected to sustained constant vertical load decreased with time, indicating that the current design method to evaluate the long-term tensile strength of geosynthetic assuming that the constant tensile load is maintained for life time is on the safe side, perhaps overly. The residual strain of reinforced sand can be made very small by preloading and further sustained loading at the preloaded state, making the effects of reinforcement stiffness negligible.

EFFECTS OF TEMPERATURE ON ELASTIC STIFFNESS OF A HDPE GEOGRID AND ITS MODEL SIMULATION

One of the important factors that affect the mechanical properties of polymer geosynthetic reinforcement is the ambient temperature. With an increase in the temperature, the rupture strength and the elastic stiffness decrease. In this study, to understand the temperature effects on the load-strain-time behaviours of a polymer geogrid, a series of tensile loading tests were performed on a high-density polyethylene (HDPE) geogrid at different but constant temperatures, and also under step-increasing temperature conditions. The test results revealed that the elastic stiffness of tested geogrid increases with the load level, while decreases with the ambient temperature. These properties were modelled based on the framework of hypo-elasticity. An existing non-linear three-component (NTC) model, which can simulate the load-strain-time behaviours of many types of polymer geogrid subjected to arbitrary loading histories (e.g., monotonic loading at different rates, creep or sustained loading, load relaxation) under a constant temperature, was modified to account for the dependency of the elastic stiffness on the load level and the temperature, as well as the dependency of the rupture strength on the temperature. The modified model can simulate very well the observed temperature effects on the elasticity of the tested geogrid.

LOAD-STRAIN-TIME BEHAVOURS OF TWO POLYMER GEOGRIDS AFFECTED BY TEMPERATURE

A special series of tensile loading tests was performed on two types of geogrid using a wide variety of load and temperature histories to evaluate the effects of ambient temperature on their load-strain-time behaviours. The applied loading schemes included monotonic loading and sustained loading under different controlled ambient temperature conditions. The followings were found from test results and their analysis performed in this study. With an increase in the ambient temperature, the rupture strength and stiffness decreased while the creep strain increased associated with a decrease in the stiffness. The creep strain by sustained loading during which the temperature was elevated from 30 o C to 50 o C was significantly larger than the one by sustained loading during otherwise monotonic loading at the constant temperature equal to either 30 o C or 50 o C. The elastic stiffness decreased with an increase in the temperature while increased with the tensile load level. Importantly, the residual tensile strength observed at the same ambient temperature was essentially independent of pre-rupture loading histories.

Correlations Between Strains in a Thin Asphalt Pavement Structure and Deflection Basins: Investigation by Linear Elastic Analysis and Experiments

In rural areas, thin asphalt surface pavement structures have been constructed for low-volume road networks. For this type of road, a light weight deflectometer (LWD) may be useful for evaluating the responses of a pavement structure. Of the responses, the mobilized horizontal tensile strain at the bottom of asphalt pavement indicates the fatigue cracking potential, while the mobilized vertical compressive strain at the top of the base layer indicates the rutting potential. A series of linear elastic analyses was performed on a thin asphalt surface pavement structure. Correlations between the two mobilized strains and deflection basin parameters were found. A model thin asphalt surface pavement structure was constructed in the field. Many sensors were installed for measuring the mobilized stresses and strains during an LWD test. The test results generally were in agreement with the linear elastic analyses. The mobilized strains in the thin asphalt surface pavement structure could be predicted with the correlations with deflection basin parameters obtained from the LWD test.

FEM analyses on creep characteristics and strain fields of geogrid-reinforced sand

ABSTRACT Both sand and polymer geogrid reinforcement are known to exhibit more-or-less complicated stress-strain-time or load-strain-time behavior including instantaneous non-linearity and viscous effects. Creep is one of the most important time-dependent behaviors of material, which is an inherent response of the viscous property of material. Due to interactions between the viscous sand and reinforcement, the creep characteristics of geogrid-reinforced sand could be very complicated. A nonlinear finite element method (FEM) analysis technique incorporating the unified three-component elasto-viscoplastic constitutive model for both sand and geogrid was developed. The FEM can simulate the whole process including the constant strain rate loadings and the creep loading stages. In addition, the development of strain fields during the creep loading can also be reproduced by the FEM simulation. By comparing the simulated results with the experimental results, it was shown that the proposed elasto-viscoplastic FEM could well simulate the creep characteristics of geogrid-reinforced sand, especially for the high stiffness following a creep loading stage.